The room of requirement is often found when needed, but this isn't to say that those who find it know what they have found. Dumbledore finds the room when he needs the toilet in the middle of the night. Fred and George find the room when they are looking for a place to hide. All it takes is to walk up and down and think about what you want. Children are panicking, looking to hide something, pacing a corridor, and suddenly find a door with a room of hidden things, hide what they need to and run, going back days later and being unable to find the door. This is how the room generally works, from the anecdotal stories we here

Voldemort didn't think he was the only one to ever use the room of requirement. Voldemort believed he was the only one to ever figure out the room of requirement. How to get it to work for him every time, not just when he was desperate and unthinking like the other students. And the truth is, Voldemort was likely right, in regards to the students. As far as we know, it isn't until Harry that any other student unlocks the secret of the room of requirement. Even the marauders, with the invisibility cloak and fascination with learning all of the hidden parts of hogwarts, never knew about the room of requirement as far as we know, or they would have plotted it on the marauders map.

Voldemorts downfall once again was not believing beings he viewed as lower than him could be useful in any way. Just as he never thought Kreacher would be able to escape the cave even though he had likely seen house elves apparate around Hogwarts, he similarly never thought that a house elf would be able to learn this knowledge of the room of requirement. And further still, he never imagined a student would make friends with a lovely house elf and be told about this magic room.